The present invention relates to a differential olfactometer. It relates to a physiological zero detector for measuring the intensities of odours of different olfactory stimuli.
It is known that the function of olfactometry is to supply an olfactory stimulus which is well defined as regards chemical composition and concentration. It uses devices able to dilute one or more odorous gases in an odourless gas, such as air or nitrogen.
Olfactometry fulfils two different requirements: the measurement of the olfactory threshold (liminal olfactometry) and the measurement of the intensity of odours (supraliminal olfactometry).
Liminal olfactometry is simple. It is in fact easy to measure a threshold. The experimental method consists of investigating the smallest concentration which the person is able to perceive with a 50% probability.
Supraliminal olfactometry is less simple. Two methods can be used:
one method consists of subjectively estimating the intensity, expressed by a figure between 0 and a maximum;
the other method consists of establishing the equalization between the experimental stimulus and a standard stimulus appropriately chosen within an intensity range, equalization taking place by successive approximations.
Another problem which is difficult to solve in olfactometry is that of dilution and the method of presenting the stimulus. The dilution can be static or dynamic, whilst the presentation of the stimulus can be active (by inhaling or smelling) or passive (by injecting into the nasal cavities of the person concerned).
Static dilution is obtained by introducing a known quantity of odorous substances into a known volume of odourless gas contained in an inert enclosure. Dynamic dilution is obtained by mixing a small odorous gaseous current with a large odourless gaseous current. By increasing the number of dilution stages a random concentration can be obtained in this way. The best results are obtained by using dynamic olfactometers with active presentation of the stimulus. In the case of supraliminal olfactometry the most accurate and reproducible results result from the equalization method.
However, even under the optimum conditions defined hereinbefore olfactometric measurements suffer from the following inconveniences:
the equipment is heavy, cumbersome, fragile and difficult to handle;
the accuracy of intensity measurements is limited by a physiological datum, the smallest perceptible intensity difference corresponding to a concentration increase of the order of 50% (or a decrease of the order of 33%.).
Reproducible results are only obtained by repeating the measurements several times on several different people in such a way as to obtain average values excluding inter-individual variations, as well as intra-individual variations. Thus, it is not possible to reliably measure the odorous intensity of a given stimulus for a given person at a given time.
In man and all animals with equal olfactory organs the sensations received by the olfactory mucosa on one side are transmitted to the olfactory bulb on the same side. The two olfactory bulbs are connected by a nervous circuit which establishes a reciprocal inhibition between them. The more one of the bulbs is activated the more it delays the response of the other. This reciprocal inhibition is at a maximum when the two stimuli are completely synchronous to within one millisecond. It creates between the two olfactory bulbs an unstable equilibrium situation, which increases the differences in inverse proportion to their size. Thus, the equality of intensity of the two stimuli can be very accurately evaluated, the system of the two bulbs then behaving in the manner of a high gain zero detector.
It is possible to provide a device which makes it possible to overcome the disadvantages referred to hereinbefore and in particular a device which can be easily operated and which gives accurate reproducible measurements. This is achieved by physiological zero detection by measuring the equality of the odorous intensities of two separate olfactory stimuli simultaneously supplied each to one side of the person's nose.
The following procedure is adopted. By means of an electronic control released by the experimenter two olfactory stimuli, one constituting a standard and the other the stimulus which is to be measured are simultaneously supplied each to one nostril of the same experimenter. The respective intensities of the stimuli are then compared by the strongest perception felt in a nostril. The intensities in each nostril are then equalized by regulating the amplitude of the stimulus to be measured compared with the standard stimulus. The two stimuli must on the one hand be strictly synchronous and on the other must be separately adjustable in amplitude. Moreover, to eliminate distortions which may be caused by an asymmetry of the nasal cavities it is necessary to proceed as for a double weighing operation, thus for example a standard stimulus E and a stimulus X, whose amplitude it is wished to adjust so that its intensity is equal to that of E. Stimulus E is firstly supplied to the right and it is equalized with an accessory stimulus R supplied to the left and which serves as a tare. Without modifying R stimulus X is then supplied to the right and is equalized with R. Thus, E and X have the same odorous intensity.
The equality of the sensations can be evaluated electro-physiologically in the animal and subjectively in man. In the first case signals collected by electrodes carefully positioned in the two olfactory bulbs are used. In the second case the side where the sensation is most pronounced is indicated verbally. The compared intensities are considered to be equal when the sensation is indicated as being "in the middle" or no stronger "on one side than the other".
A differential olfactometer permitting the performance of supraliminal olfactometric measurements is known and which comprises:
two odour injectors with atomizers mounted on a nasal centering support;
a retractable antidiffusion device which hides the atomizers between stimulation periods;
an electronic control unit and a breathing detection device for controlling the atomizers.
Each injector has an atomizer which is mounted on a reservoir deformable under the action of an elastic transmission controlled by a motor.
Such a differential olfactometer makes it possible to perform supraliminal olfactometric measurements, but has the disadvantage of a pulsed control of the supply to the atomizers from a deformable reservoir and which is required to act synchronously with a retraction control of the antidiffusion device. Moreover the control of the antidiffusion device is realised via the breathing detection device comprising a single sensor positioned in proximity to one of the atomizers. Thus, in the case of pronounced assymmetry in the inhaling power of the nostrils of the experimenter the injection of the olfactory stimuli is not initiated at the most appropriate time for the measurements to be performed.